G Protein inhibition of Ca channels results in a positive shift in the voltage dependence and a decrease in activation kinetics, and is reversed after a strong depolarization which results in facilitation of the current. Recently, cleavage of the SNARE protein syntaxin with botulinum toxin was shown to prevent G protein modulation of presynaptic Ca channels. Additionally, Ca/calmodulin regulation of P/Q-type Ca channels gives rise to paired-pulse facilitation, similar to the depolarization-dependent relief of G protein inhibition which also results in facilitation. These results suggest that there may be convergent modulation of P/Q-type Ca channels by G proteins, SNARE proteins, and Ca/calmodulin. To test this hypothesis my specific aims are i) Determine whether SNARE protein binding to the synaptic protein interaction (synprint) site on P/Q-type Ca channels is necessary for G protein modulation. 2) Determine whether Ca/calmodulin regulation of Ca channel activity interacts with G protein modulation. 3) Determine which Ca channel G protein binding domain is necessary for regulation by either SNARE proteins and/or Ca/calmodulin. Deletion mutants of P/Q-type Ca channels, lacking either the synprint site, calmodulin binding domain, or one of the G protein binding domains, will be transfected and electrophysiologically assayed to identify the site of interactions and effects between individual SNARE proteins and/or Ca/calmodulin on G protein modulation of Ca channel function. The results of these experiments will help define the physiological consequences of these separate inputs onto presynaptic Ca channels that control neurotransmitter release at the nerve terminal, and may aid in the understanding of synaptic plasticity.